The present disclosure relates to image analysis, and more particularly to identifying the center of mass in an image.
Tracking of an item of interest in the image is desirable in various imaging applications. For example, in controlling the position of a camera for star tracking, the camera may adjust to center on a star. Similarly, it may be desirable for a camera for video conferencing to track a moving object such as a speaker. In order to track the item, an imaging system typically analyzes a captured frame of image data to determine the location of the center of mass (xe2x80x9cCOMxe2x80x9d) or centroid of the image. In an image, the COM is the location in the image which corresponds to the average light intensity, weighted by location. The camera then typically tracks that COM location.
In imaging systems which use an active pixel sensor (xe2x80x9cAPSxe2x80x9d) for image capture, the APS is typically fabricated with a CMOS process. However, conventional CMOS sensor imaging systems which identify the COM are typically limited in their imaging capability.
In research by Shibata, capacitor scaling was used for identifying the COM in a relatively small number of inputs, such as 25 inputs. However, capacitor scaling for a large number of inputs, such as in a large image, using this technique is not very practical. In research by Deweerth and Mead, a 1D photoreceptor array which computed the COM in an analog circuit was introduced. Deweerth proposed a 2D extension of this array in which the receptors were alternated spatially so that the currents from adjacent receptors were added to opposing axes. The resolution was limited in the 2D case to be half of that of the 1D case. In addition, the circuitry for calculating the COM resided in each pixel and contributed to both axes. Thus, the fill factor was lower than in the 1D case.
Furthermore, the circuit only tracked the COM, regular readout imaging was not available through the same imager.
The present disclosure describes apparatus and techniques for identifying the location of the center of mass (xe2x80x9cCOMxe2x80x9d) in an image. In one aspect, an imaging system includes a plurality of photosensitive elements arranged in a matrix of M columns and N rows, where M greater than 1 and N greater than 1; a center of mass circuit coupled to the photosensitive elements, including a resistive network and a normalization circuit including at least one bipolar transistor. The center of mass circuit identifies a center of mass location in the matrix and includes: a row circuit, where the row circuit identifies a center of mass row value in each row of the matrix and identifies a row intensity for each row; a horizontal circuit, where the horizontal circuit identifies a center of mass horizontal value relative to the matrix based upon the center of mass row values, such that the center of mass horizontal value indicates the horizontal coordinate of the center of mass location; and a vertical circuit, where the vertical circuit identifies a center of mass vertical value relative to the matrix based upon the row intensities, such that the center of mass vertical value indicates the vertical coordinate of the center of mass location.
An advantage of the techniques described herein is that the circuitry for identifying the COM is provided on the periphery of the imager so that regular imaging is possible without degradation from the COM circuitry. Also, the COM may be identified without reading out the entire frame of image data.